Have you checked the output by your dvd player? I too had the same problem a couple of months back...CD sound, radio, etc was all up to par, but then when I'd play a DVD, it was significantly lower. What I found out was, though I had my receiver set up to play in surround mode, the DVD player itself wasn't sending surround information...it was still set to send analog signals...why that was lower, I don't know. At any rate, I went to my DVD players instruction manual and found that, by default, it was set on analog...if I wanted dolby, I had to go into the user functions and actually set it to send surround info... I guess they do that for the folks who aren't planning have surround and just have the DVD player hooked to the back of their tv. After setting it to send Dolby info, it was significantly louder.
I don't know if this is, in fact, what's going on with yours, but it's a start...your problem sounds similar and this is what fixed mine. Hope it helps.